#mass spectrometry

Mass Spectroscopy

In peptide analysis, which cleavage forms b/y ions?

Peptide backbone contains three bonds repeating per residue: N-Cα, Cα-C carbonyl, and C-N amide peptide bond. Fragmentation nomenclature associates specific bond cleavage with ion type. Breaking the amide linkage CO-NH retains N-terminal portion as b ion and C-terminal portion as y ion, most common pathway under collision-induced dissociation due to relative bond weakness and proton mobility mechanism. Cleavage of N-Cα produces c/z ions characteristic of electron transfer dissociation, while Cα-CO yields a/x ions. Thus CO-NH scission directly generates b/y series used for sequence determination in proteomics data analysis routinely performed.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

Which MS mode shows a single peak for intact peptide?

Electrospray ionization inherently produces distribution of charge states due to multiple proton attachment, leading to envelope of peaks for single protein species. Tandem mass spectrometry deliberately fragments ions giving many product peaks. Time-of-flight is an analyzer, not ionization mode. MALDI predominantly generates singly protonated species [M+H]+ because matrix-assisted desorption transfers single proton without extensive multiple charging. Consequently, MALDI spectrum of intact peptide typically shows one major monoisotopic peak cluster representing intact molecular weight, simplifying interpretation of purified peptides and enabling rapid assessment of purity, unlike ESI where deconvolution becomes necessary for molecular weight determination complexity.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

Which modification causes +28 Da shift?

Post-translational modifications exhibit characteristic mass increments in mass spectrometry analysis. Formylation introduces a formyl group –CHO to lysine or N-terminus through formyl phosphate donor or chemical artifact, adding CO moiety essentially 27.995 Da rounded to +28 Da to peptide mass. This shift must be distinguished from dimethylation or ethylation. Sulfonation adds 80 Da, methylation adds 14 Da, phosphorylation adds 80 Da. Detection of +28 Da suggests formylation event often observed as artifact in formaldehyde crosslinking or as biologically relevant histone modification influencing protein function assessed in high-resolution accurate-mass protein studies carefully evaluated today.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

A shift of 98 Da less than expected in MS/MS means:

Phosphorylation adds 80 Da, but under collision-induced dissociation phosphorylated serine and threonine frequently undergo beta-elimination of phosphoric acid H3PO4 with mass 97.9769 Da, rounded to 98 Da loss from precursor or product ions. Observation of peak 98 Da lower than predicted molecular weight signals neutral loss of phosphate, indicating presence of phosphopeptide that lost H3PO4 during fragmentation. This diagnostic loss distinguishes phosphorylated peptides from unmodified counterparts and contrasts with oxidation or deamination shifts. Recognition of -98 Da loss supports phosphosite localization in signal transduction studies and proteomics research widely used.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

What happens in fast atom bombardment?

Fast atom bombardment, introduced before electrospray and MALDI, ionizes nonvolatile biomolecules by embedding sample in viscous glycerol matrix placed on a metal target. A beam of high-energy argon or xenon atoms accelerated to kiloelectronvolt energies bombards surface, sputtering analyte molecules with protonation and desorption into gas phase. Resulting ions show minimal fragmentation, enabling measurement of peptides and small proteins. The technique relies on momentum transfer from neutral atoms rather than electron ionization, matrix desorption by laser, or X-ray irradiation, representing an early soft ionization approach for biological mass spectrometry applications historically significant.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

What kind of fragments dominate in peptide MS spectra?

During low-energy collision-induced dissociation of protonated peptides, proton mobility model dictates that proton migrates along backbone and weakens peptide bonds, producing mainly b and y type ions. Among these, y ions containing C-terminal portion often dominate spectra because N-terminal fragments may further fragment or undergo cyclization. C-terminal lysine or arginine in tryptic peptides strongly sequesters proton, enhancing stability of y ions. While a-ions accompany b ions, x-ions are rare in CID, and c/z ions appear in electron-based dissociation methods, making y-ion series most informative for database searching in proteomics experiments.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

What is the main principle of TOF detection in MS?

Time-of-flight analyzer imparts equal kinetic energy to ions through high voltage acceleration into a field-free drift region. Kinetic energy equals zV = 1/2 mv², so velocity equals sqrt(2zV/m), making flight time proportional to sqrt(m/z). Thus lighter ions with lower m/z travel faster and arrive earlier at detector than heavier ions. Calibration converts measured time to accurate m/z values. Statement that all ions have same time or intensity proportional to energy is incorrect. This dependence forms basis for high-resolution analysis of proteins, peptides, and oligonucleotides over wide mass range without scanning.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

Which technique can sequence a 25-residue peptide directly?

Determining amino acid order of a 25-residue peptide requires fragmentation along peptide backbone to generate overlapping ions. Circular dichroism assesses secondary structure, SDS-PAGE estimates molecular size, and X-ray diffraction requires crystals unsuitable for short peptides. Tandem mass spectrometry, MS/MS, isolates chosen precursor ion and fragments it via collision-induced dissociation, producing b and y ion series covering sequence. Modern Orbitrap or quadrupole-time-of-flight instruments achieve sufficient resolution and mass accuracy to read full sequence directly, enabling rapid peptide sequencing without Edman degradation in proteomics workflows for biological samples.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

A mass shift of +203 Da suggests presence of:

Glycosylation adds monosaccharide residues with defined masses to proteins, altering function and stability. N-acetyl glucosamine, GlcNAc, exists as common modification in N-linked and O-linked glycans and O-GlcNAc signaling. Residue mass of HexNAc after dehydration during glycosidic bond formation is 203.079 Da, observed as +203 Da increment on peptide mass spectra. Detecting this shift indicates single GlcNAc attachment, distinct from 162 Da for hexose, 146 Da for fucose, or 291 Da for sialic acid. Recognition of +203 Da aids mapping of glycosylation sites in proteomics studies.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

Which property is essential for MALDI matrix?

MALDI requires matrix compounds that isolate analyte molecules, absorb energy from pulsed laser typically 337 nm or 355 nm, and facilitate proton transfer. Strong ultraviolet absorption ensures efficient energy uptake, leading to rapid sublimation of matrix crystals and entrainment of analyte into gas phase without direct photochemical damage. High volatility would evaporate prematurely, non-reactivity alone insufficient, and fluorescence indicates emission rather than absorption. Common matrices like CHCA, DHB, and sinapinic acid all possess aromatic systems with high molar absorptivity at laser wavelength, enabling gentle desorption and ionization of peptides.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

Which is a soft ionization technique?

Ionization techniques are categorized as hard or soft based on extent of fragmentation induced. Hard ionization like electron ionization deposits abundant internal energy causing extensive bond cleavage. Soft methods preserve molecular ions. MALDI uses matrix buffer absorbing laser energy, producing intact singly charged ions. Fast atom bombardment bombards analyte in glycerol matrix with high-energy argon atoms, also yielding protonated molecules with little fragmentation, enabling analysis of polar peptides. Electrospray is also soft. Therefore, both MALDI and FAB qualify as soft ionization techniques, while electron ionization does not meet this gentle energy deposition criteria.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Mass Spectroscopy

A shift of +42 Da in MS indicates:

Lysine residues undergo enzymatic acetylation where an acetyl group CH3CO replaces hydrogen on epsilon amino group, neutralizing positive charge and regulating chromatin structure and metabolism. Addition of C2H2O minus H corresponds to monoisotopic mass increment of 42.0106 Da, observed as +42 Da shift in mass spectra. Detecting this modification on peptides helps identify epigenetic regulatory marks and metabolic regulation. Differentiation from trimethylation which also adds ~42 Da requires high resolution or diagnostic fragment ions, but in typical low-resolution proteomics workflows +42 Da strongly suggests acetylation event on proteins.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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